“A similar process goes on in mortar: the lime takes back from the atmosphere the gas that it had lost in the heat of the lime-kiln, and little by little becomes stone again. The sand mixed with it serves to disintegrate the lime, which thus more easily absorbs the air necessary for its conversion into limestone. When the mortar has fully resumed the form of limestone the courses of masonry are so strongly bound one to another that the stones themselves sometimes break rather than give way.
“What is known as fat lime is lime that develops great heat when brought into contact with water, and also increases considerably in volume, forming with [[51]]the water a thick, cohesive paste. On the other hand, poor lime develops but little heat, disintegrates slowly, and increases scarcely any in volume. The first kind comes from nearly pure limestone and can be mixed with a large proportion of sand, thus making a great quantity of mortar. The second kind is obtained from limestone having various foreign substances and will admit of but a small admixture of sand, thus yielding less mortar than the other. Both have the property of hardening in the air by the absorption of carbonic acid gas which converts them into limestone.
“There is a third variety of lime called hydraulic lime, which has the peculiar merit of being able to harden under water. It is made from a limestone containing a certain proportion of clay. Hydraulic mortar is used for the masonry of bridges, canals, cisterns, foundations, vaults, in fact for all stone and brick work under water or in damp soil.” [[52]]
CHAPTER X
LIME IN AGRICULTURE
“To be fertile a soil must contain limestone, sand, and clay, besides the organic substances coming from humus and fertilizers. Now it may be that nature has not endowed the soil with a sufficient quantity or with any of these three constituents. Then the character of the soil must be corrected by giving it what it lacks. That is what is called improving the land. Thus a soil that is too sandy is improved by the addition of limestone and clay; one that is too compact, too clayey, is improved by adding sand and, still more, by adding limestone. Mineral substances thus added to the soil to correct it are called correctives. These substances coöperate also in the nutrition of plants, and from this point of view may be regarded as mineral fertilizers.
“One of the most valuable of correctives is lime, which is indispensable to soils lacking limestone, indispensable also to the nutrition of nearly all our cultivated vegetables. It acts in various ways. First, it energetically attacks vegetable substances, decomposing them and converting them into humus. A pile of leaves that would take long months to rot becomes in a short time a mass of humus when mixed with lime. Hence its great utility in fields overgrown [[53]]with weeds, and in newly cleared land—in short, wherever there are old stumps, piles of leaves, remnants of wood, and patches of heather, which need to be decomposed. With the help of lime all these herbaceous or woody substances are quickly converted into humus, with which the soil becomes enriched to the great advantage of future crops.
“In the second place, lime corrects or neutralizes the acidity peculiar to certain soils, as is proved by the following experiment. Let us mix some vinegar, no matter how strong, with a little lime. In a short time the smell and acid taste of the vinegar will have disappeared. Now wherever masses of vegetable refuse, such as leaves, mosses, rushes, old stumps, are undergoing decay, there are produced certain sour-tasting substances or, in other words, acids, which are invariably harmful to agriculture. This generation of acid occurs notably in turfy soils, which have an excessive acidity favorable to the growth of coarse rushes and sedges that are valueless to us, and at the same time this acid is highly injurious to all our cultivated plants. Lime, therefore, which is sure to correct this acidity, works wonders in marshy lands, damp meadows, and turfy soils. We are warned of the need of lime by the appearance of ferns, heather, sedge or reed-grass, rushes, mosses and sphagnei.
“Thirdly, when once mixed with the soil, lime speedily resumes the form it wore before passing through the lime-kiln; that is to say, it becomes limestone, but in the shape of fine powder. This return [[54]]to the limestone condition is brought about by union with the carbonic acid gas coming from the atmosphere or thrown off by the substances decaying in the ground. Under this new form lime continues to play a useful part by supplying the calcareous ingredient to soil that lacked it, and also by preventing the clay from becoming too cohesive, too impervious to air and water.